Battery power source devices of this type are known in which a plurality of single batteries including secondary batteries are electrically and mechanically connected in series to constitute battery modules, a required output voltage being obtained by series connection of a prescribed number of such battery modules accommodated in a holder case, these being capable of being freely charged or discharged.
However, since a battery power source device constructed as described above is exposed to severe conditions of use when carried in an electric automobile or hybrid vehicle, maintaining it in a suitable operating condition by controlling the temperature and/or charging/discharging of the battery etc. is demanded. In order to meet this demand, it is necessary to know the condition of the battery power source device itself by detecting the temperature and/or voltage etc. of the battery power source and to report to the external device that uses the battery power source data indicating the operating condition of the battery power source, such as the detected temperature, voltage, current and SOC (State Of Charge), which indicates the amount of electricity stored relative to the battery capacity at 25.degree. C.